Many forms of nucleic acid amplification reactions have been developed in recent years. The first method was the Polymerase Chain Reaction (PCR) which involved repeated cycles/ of heating to separate the DNA strands, primer annealing to the strands, and primer extension by a DNA polymerase. Product accumulation from the PCR reaction is exponential; that is, the amount of product doubles for every cycle of amplification. Therefore, the expected amount of product may be calculated by the formula (Eff*2)n where Eff is the efficiency of the primer annealing and primer extension reaction, and n is the number of cycles.
An alternative method for target amplification was developed called NASBA (Nucleic Acid Sequence Based Amplification) (see e.g., Compton (1991) Nature 350: 91–92). This method relies on the concerted action of three enzymatic activities, Reverse transcriptase, RNaseH, and RNA Polymerase, to amplify an RNA target. Reverse transcriptases generally possess an endogenous RNase H activity, which can under the correct conditions, substitute for exogenously added RNase H activity. Primers are first designed which have an RNA polymerase site together with a target recognition sequence. Then, the primers are added to the target nucleic acid together with the three enzyme activities. First, primer binds followed by primer extension across the sequence of interest. The result is a double-stranded RNA-DNA hybrid. The RNA portion of the hybrid is digested by the RNase H activity allowing binding of the other primer. The reverse transcriptase activity then extends this primer back across the sequence of interest finishing at the RNA polymerase binding sequence. The RNA polymerase activity then transcribes the sequence of interest making multiple single-stranded RNA copies. These RNAs may bind more primers and the cycle continues. Because each transcription step yields 10–100 copies of RNA per copy of template, product accumulates rapidly and logarithmically.
Still, another method has been developed which is called SDA or strand Displacement Amplification (see e.g., Walker (1993) PCR Meth. Appl. 3: 1–6. This method utilizes four primer sequences with two primers binding on either end of the sequence of interest. It also requires a DNA polymerase and a restriction endonuclease (A restriction endonuclease binds to a specific sequence called its recognition site, and then cleaves the DNA a specific sequence). In the first step, nucleic acid strands are heat separated allowing the binding of the first primer pair. The inner primer contains a restriction enzyme site which is non-complementary to the target sequence, while the outer primer binds just upstream of the inner primer. DNA polymerase extends both primers, but extension from the outer primer displaces the newly synthesized inner strand yielding a single strand template for primer binding. Extension reactions are done in the presence of a nucleotide analog (alpha-thio-dATP such that the newly synthesized strands are fully substituted making them immune to cleavage by the restriction endonuclease. However, since the inner primers are not substituted, and the complement of the inner primer is substituted, the restriction enzyme will create a nick within the inner primer sequence by cutting only within the unsubstituted sequence. The nick can act as a priming site for DNA polymerase. In the process of extending the nick, the DNA strands are separated or displaced by the DNA polymerase creating single strand primers which can then bind inner primers for the next round of amplification. Accumulation of product for SDA is therefore exponential since every priming event doubles the amount of product.
Other amplification schemes have been devised, but they all require generating a single strand intermediate that allows primer binding for continued rounds of amplification (see e.g., Fahy et al. (1991) PCR Meth. Appl. 1: 25–33; Guatelli et al. (1990) Proc. Nat. Acad. Sci., U.S.A. 87: 1874–1878. While the methods described above have been shown to work well, they do have some drawbacks. PCR requires the use of a thermocycler to obtain rounds of strand separation and primer extension. Furthermore, the process of heating and cooling can be slow resulting in a PCR reaction requiring a few hours to complete from start to finish. NASBA circumvents this issue by being run isothermally, that is at a single temperature. The products are single-stranded RNA which can be relatively unstable especially if an RNase activity, which are ubiquitous, is inadvertently introduced. RNA products are also generally chemically less stable. Furthermore, the length of the expected product dictates the efficiency of the amplification reaction. This is in part due to the RNAaseH activity which can inactivate RNA transcripts with bound primers. NASBA reactions also require the addition of high concentrations of both ribonucleotides and deoxyribonucleotides increasing the cost of running a reaction. NASBA reactions are also run at lower temperatures leading to the production of spurious amplification products. In SDA, while the amplification products are DNA, the products are modified by the presence of the alpha-thio-dATP used to inhibit strand cleavage by the restriction endonuclease which may make further manipulation of the product difficult, especially in research applications.
There is a need for improved methods of nucleic amplification. This invention meets those needs.